Carbon fiber composites wherein carbon fibers are included in a synthetic resin matrix are well known in the art. The function of the resin matrix is primarily to give the composite the required external dimension and shape, while the carbon fibers provide the composite with the required strength and stiffness.
Thermoplastic polymers, which are solids at the intended use temperature of the composite such as polyesters, have been employed as the resin matrix as illustrated by British patent specification No. 1,228,573. Thermoplastic resins possess the advantage of not requiring partial curing after impregnation of the carbon fibers. When a thermoplastic polymer, such as polybutylene terephthalate, is employed as the resin matrix, however, the bonding between the carbon fiber and matrix is often poor as manifested by the mechanical strength properties, e.g., interlaminar shear strength of the composite.
Improved bonding between the carbon fibers and matrix resins in general has been accomplished in the past by a number of techniques such as by plating the carbon fibers with various metals including tantalum, metal carbides and nitrates, and by etching the surface of the carbon fibers with oxidizing agents such as air, ozone, concentrated nitric acid and chromic-sulfuric acid. Block and graft copolymers have also been used to improve bonding between the carbon fibers and the matrix as illustrated by U.S. Pat. No. 3,855,174. Carbon fibers have also been coated with a variety of sizing agents such as epoxy resins for a number of reasons, such as to prevent snagging and fraying of the carbon fibers as illustrated by U.S. Pat. Nos. 3,373,984; 3,806,489; 3,908,042; 3,914,504; 3,953,641; 3,971,669; 4,145,472 and British Patent Specification No. 1,195,219.
When the epoxy resins typically employed as sizing agents such as the diglycidyl ethers of Bisphenol A are employed in making carbon fiber composites wherein the matrix resin is polybutylene terephthalate, however, the interlaminar shear strength of said composites are not as high as one would desire to meet certain practical requirements of the carbon fiber composites.
It is therefore an object of the present invention to improve the bond between the carbon fiber surfaces and certain thermoplastic polyesters employed as the resin matrix.
It is another object of the present invention to provide carbon fibers coated with a sizing agent which can be employed in combination with certain thermoplastic polyester matrix resins to prepare carbon fiber composites which exhibit improved interlaminar shear strength and acceptable flexural strength.
It is a further object of the present invention to provide a carbon fiber composite with enhanced interlaminar shear strength and acceptable flexural strength.
These and other objects and features of the invention will become apparent from the claims and from the following summary and description of the preferred embodiments of the present invention.